In conjunction with the technical advance and increasing demand of mobile tools, the demand for nonaqueous electrolyte secondary batteries as the energy source is in rapid growth. Among others, research works are focused on lithium secondary batteries because of their high energy density and high discharge voltage.
In general, the nonaqueous electrolyte secondary battery is constructed of a positive electrode, a negative electrode, and a separator interposed therebetween. Such a secondary battery has the problem that if the water content inside the battery increases, the electrolyte is degraded thereby to generate acid. The acid thus formed promotes side reactions, for example, degradation of solid electrolyte interface at the negative electrode, and dissolution of positive electrode active material. There eventually arise problems like a lowering of battery capacity and an increase of internal resistance.
In this regard, Patent Document 1 discloses the technique of admixing hydrophobized inorganic powder into the separator. Generally the separator material itself is hydrophobic. Thus this technique of imparting hydrophobicity to the separator material is not necessary insofar as no hydrophilic inorganic substance is further added to the separator material.
Patent Document 2 discloses the technique of coating an electrode material to an electrode current collector, and forming a layer of hydrophobic material on the surface of the electrode, for thereby preventing water from penetrating into the electrode material. With this technique, after the electrode material is coated onto the electrode current collector, the hydrophobic material is coated on the surface of the electrode material to form a layer which is effective for preventing water penetration. Accordingly, while water can be absorbed in or enter the electrode material during the preparation or coating of the electrode material, it is substantially impossible to evaporate off this water because of the overlying hydrophobic material layer.
Patent Document 3 proposes the technique of removing water contained in electrode material. An electrode material is coated onto an electrode current collector and then sintered. With this technique, however, various additives to construct the electrode material, for example, binder can undergo decomposition reaction during sintering. Accordingly, it is not preferable to apply this technique to the battery manufacture.
Patent Document 4 discloses addition of hydrophobic inactive particles to the electrode material itself. The hydrophobizing method is by treating ordinary hydrophilic inorganic oxide with a hydrophobic substance (e.g., hydrophobic silane compound) while the particle size and shape of the resulting hydrophobic inert particles are not specified. A simple mixture of the hydrophobic inactive particles (in Patent Document 4) with the electrode material is difficult to efficiently inhibit water entry or water-aided side reactions because the inorganic oxide particles are poorly dispersible. Even when entry of water from the exterior is prevented, it is difficult to effectively remove water and moisture from within the electrode material.